Water-resistant supports both sides of which are coated with polyethylene have already been commercialized with a view to enabling rapid processing of photographic paper. The currently manufactured and commercially avalable types of photographic paper with a water-resistant support have either a glossy surface or a textured (e.g. matted or silk-finished) surface. The surface configuration or contour of such photographic paper directly reflects the pattern formed on the polyethylene-coated surface of the support on the photographic emulsion side. While various surface configurations are known, a "glossy surface" which is smooth and is not textured at all is most popular and photographic paper with a glossy surface is being sold in larger quantities than any other types of paper. Preferably, photographic paper with a glossy surface has a specularly smooth surface, with a minimum number of fine asperities being present on the image-carrying side.
The present invention relates to a support for photographic paper having improved surface smoothness. The conventional photographic paper using baryta paper as a support is provided with a virtually specular surface by being "ferrotyped" in the drying step which is the final stage of processing. On the other hand, photographic paper using a water-resistant support is finished without being "ferrotyped", so the smoothness of the image-carrying side directly reflects the surface of complete photographic paper which is coated with a photographic emulsion. Since the thickness of the emulsion layer in photographic paper is as small as about 10 microns, the smoothness of the surface of the photographic paper can be regarded as completely identical to that of the support. Therefore, in order to attain an image-carrying side having improved smoothness, the surface of the support must be as smooth as possible.
In producing a water-resistant support, a polyethylene layer is applied most commonly by extrusion coating wherein molten polyethylene is cast onto the surface of a travelling base paper. The present invention relates to a water-resistant support having improved smoothness attained by extrusion coating. It is known that the smoothness of the surface of polyethylene coated paper produced by extrusion coating can be improved by several methods such as increasing the thickness of the polyethylene coat and exerting a greater force of compression during polyethylene coating. But these methods are not highly effective and the cost of their operation is quite high.
The most common method for improving the smoothness of the base paper is to increase its density by exerting a greater pressure in machine calendering, but even this method has the disadvantage that the appearance of the paper is prone to be impaired by such defects as blackening and cockle formation. Therefore, none of the methods so far proposed has succeeded in achieving satisfactory smoothness in the surface of the suppport of photographic paper.